Drilling fluids (or drilling muds) are used in the process of drilling wellbores. The drilling fluids are circulated through the wellbore during drilling operations to remove cuttings from the wellbore and to lubricate the drill bit. Drilling fluids are also used to maintain a sufficient hydrostatic head in the wellbore to prevent blowouts by balancing the pore pressure of the formation. Many drilling fluids are invert emulsions including a continuous phase formed of a base oil and an internal phase formed of an aqueous solution. Emulsifiers are included in drilling fluids for stabilizing the interface between the continuous phase and the internal phase. Other additives, such as weighting agents, are generally included in drilling fluids.
The rheology of drilling fluids involves an analysis of shear stress, shear rate, and viscosity. Viscosity is defined as the ratio of shear stress to shear rate where shear stress is the force per area (typically expressed in N/m) and the shear rate is the change in velocity over distance. When a fluid begins to flow under the action of a force, a shearing stress opposing the motion arises throughout the fluid. As one layer of fluid moves past an adjacent layer, molecules interact to transmit momentum from the faster layer to the slower layer, thereby resisting the relative motion. Hence, a distinguishing feature of fluids in contrast to solids is the ease with which fluids may be deformed under an applied force. It is the fluid's viscosity that creates resistance to this force. Either shear stress or shear rate must be controlled while the other is measured under well-defined conditions to acquire an accurate viscosity measurement. Viscosity data often functions as a window through which other characteristics of a material may be observed.
Drilling wells in deepwater can result in a three to fourfold increase in the viscosity of conventional invert-emulsion drilling fluids. An invert-emulsion drilling fluid is typically more viscous at the seabed and does not flow easily because the drilling fluid temperature is reduced by the deepwater environment. The increased resistance to flow can increase the fluid column hydraulic pressure when circulating the drilling fluid. Increases in column pressure can overcome the wellbore horizontal stress and exceed the fracture gradient, which can result in loss of drilling fluid (i.e., loss of circulation) to induced fractures in the formation. Another potential viscosity increase occurs when drilling operations and circulation of the drilling fluid are stopped, particularly if the mud system remains static over time. When static, the drilling fluid can develop a gel strength that may require high pumping pressure to reestablish circulation. Effects of increased viscosity include costly loss of drilling fluid, severe reservoir damage, and loss of wellbore integrity.
Flat rheology drilling fluids have been developed for deepwater wells in an effort to prevent loss of circulating drilling fluid to induced fractures. Flat rheology drilling fluids are designed to demonstrate a minimal variance in certain rheological properties across temperatures from 40° F. to 150° F. as measured at atmospheric pressure. Relevant rheological properties include plastic viscosity (“PV”), yield stress, yield point (“YP”), low shear yield point (“LSYP”), and gel strength.
Current methods for designing flat rheology drilling fluids define variations in rheological properties only under atmospheric pressure conditions, not downhole pressures. In fact, the rheological properties of the flat rheology drilling fluids have been shown to change under downhole versus ambient conditions, especially in deepwater environments.